A solar cell is a semiconductor device for converting solar energy into electric energy. It has been known that, after E. Becquerel discovered the photoelectric effect in 1839 for the first time, a Se cell with 1 to 2% efficiency was first used for an exposure device of a camera through studies on the photoelectric effect of Se by H. Hertz in 1870. Thereafter, the solar cell was used in military equipment such as a power source of a satellite in the late 1950's, during the initial stage of development. However, commercialization has led to rapid progress since various countries invested immense research funds for the purpose of using the solar cell as alternative energy under the influence of the oil shock in 1970's.
A solar cell uses the photovoltaic effect of a semiconductor, and manufactured by combining p-type and n-type semiconductors. If light is incident onto a portion (p-n junction) where the p-type and n-type semiconductors are joined together, negative charges (electrons) and positive charges (holes) are produced by light energy within a semiconductor. Theses electrons and holes move to n-type and p-type semiconductor layers while being separated at an energy barrier interposed there between such that they are gathered to both electrodes, respectively. Since, if such two electrodes are connected through a conducting wire, a current flows, it can be used as electric power outside.
FIG. 1 is a view showing a structure of a general semiconductor solar cell. The structure of the solar cell includes: a p-n junction structure in which p-type and n-type semiconductors that can be easily seen in a diode, LED or the like are joined together; upper and lower electrodes 11 and 15; and an antireflection (AR) layer 12 for reducing the reflection loss of and light. If a semiconductor absorbs light (photons) due to the photoelectric effect in view of a property of the semiconductor, free electrons and holes are produced, and photon energy absorbed while these free electrons and holes are being recombined is converted into photon energy such as heat in a general semiconductor. However, the positions of the free electrons and holes existing in the vicinity of the p-n junction are changed with each other due to an electromagnetic field around the p-n junction so that an electric potential is formed in a solar cell. As a result, if an external device is connected to the solar cell, a current flows.
However, such a solar cell has a problem in that its efficiency is currently low and it is a high-priced device. Actually, the primary problem that should be solved for the purpose of commercializing the solar cell is to enhance the efficiency of the solar cell. One of reasons for the low efficiency caused by the solar cell is covering loss by the upper electrode 11 thereof. The solar cell provided with a p-n junction structure should perform metal bonding to respectively form electrodes on both surfaces of the junction. However, since metal is generally an opaque substance, light is not transmitted through the bonding portion, and an area as broad as the bonding portion is not used so that efficiency is lowered. If the metal bonding is accomplished only on a portion of the junction surface in order to extend an area through which light is transmitted, the resistance of the solar cell is increased because the resistance of silicon itself practically used as a substrate for a p-n junction diode is large. Such an increase of the resistance increases energy loss. Thus, the aforementioned problem can be improved using buried bonding.
FIG. 2a is a view showing a section of a buried contact solar cell. A buried contact is formed by engraving a groove on a top of the solar cell using a laser or a mechanical method, and then forming a metal electrode 23a in the groove. In a case where such a method is used, there is an advantage in that the contact resistance and covering loss of the solar cell are simultaneously reduced, and there is benefit of the reduction effect of a dead cell through a partial doping, the enhancement of a light response property in a short wavelength band, and the like. However, the aforementioned metal bonding formation of the buried contact generally uses a squeezee method or electroplating method. Since there is a problem of the uniformity of paste amount for forming a metal wiring in a case where the squeezee method is used, there is a problem in that a metal substance filled with a groove 23b protrudes outside the groove so that the security of a fixed line width is difficult and even the metal wiring is cut (see FIG. 2b). Further, since a seed metal is formed through electroless plating before electroplating, and then the electroplating additionally progresses in a case where the electroplating is used, there is a problem in that the efficiency of a process is low, and a throughput is lowered due to a low electrodeposition speed in a case of mass production.